1. Field of the Invention
This invention relates to processes for improving the melt flow rate, tack and cold flow properties of ethylene-vinyl acetate resins, and more particularly, to a catalytically induced oxidative toughening of an ethylene-vinyl acetate copolymer.
2. Description of the Prior Art
Ethylene and vinyl acetate (EVA) copolymers are well known items of commerce having a wide variety of applications. EVA copolymers containing from a small amount up to about 70 weight percent combined vinyl acetate are conveniently produced by any of the known and conventional high pressure bulk or solution copolymerization techniques. EVA copolymers containing more than about 70 weight percent combined vinyl acetate are advantageously produced by low pressure emulsion copolymerization processes whose operational parameters are well known. EVA copolymers having a combined vinyl acetate content in the range of about 35 to about 70 weight percent are of particular interest due to their rubbery or elastomeric nature. However, EVA copolymers produced via the aforementioned high pressure processes tend to have relatively high melt flow rate (low molecular weight), resulting in products in the rubbery composition range which have undesirable tack and cold flow properties. And, while EVA copolymers produced by the aforementioned low pressure processes have desirably low melt flow rate, such processes are unsuitable for the production of rubbery EVA copolymers and tend also to yield undesirably high gel fractions.
Processes are known wherein EVA copolymers of high melt flow rate can be cross-linked to provide resins of reduced melt flow rate.
U.S. Pat. No. 3,491,075 describes a process for cross-linking EVA copolymers and other types of resins employing alkoxides (alcoholates) of the general formula M(OR)n wherein M is a polyvalent metal of Groups II, III, IV, V, VII and VIII, and R is an alkyl group having from 1 to about 20 carbons and n has a value of 2 to 5 as cross-linking agents. The by-product ester which is produced during the cross-linking reaction can be removed from the copolymer if desired by vaporization. Cross-linking of the resin is evidenced by a higher softening point than the original copolymer.
British Pat. No. 1,198,877 describes a process for cross-linking EVA copolymers wherein the copolymers are heated to a temperature between 60.degree. and 250.degree. C. in the presence of an alkali metal alcoholate as cross-linking agent. Cross-linking results in a reduction in the melt index. For example, an EVA copolymer containing 13.5 percent by weight combined vinyl acetate and a melt index of 2.5 is cross-linked in a Brabender Plastograph using lithium n-octylate to provide a copolymer of 0.4 melt index. In the cross-linking reaction, an ester is formed which may or may not be removed from the cross-linked copolymers. This patent also contemplates conducting the alcoholate cross-linking reaction simultaneously with, or subsequent to, cross-linking with peroxide. The addition of an oxidation stabilizer during the alcoholate cross-linking reaction is also described.
In the similar EVA copolymer cross-linking process described in British Pat. No. 1,198,879, alkoxo salts of the general formula Me.sup.1 [Me.sup.2 (OR).sub.x ] in which Me.sup.1 is hydrogen or a mono or bivalent metal, Me.sup.2 is a different mono or pentavalent metal or silicon, x has a value between 2 and 6 and the R groups are the same or different aliphatic or cycloaliphatic groups are used as the cross-linking agent. An ester is also formed in the course of the cross-linking reaction which may or may not be separated from the cross-linked resin.
East German Pat. No. 94,489 describes a process for cross-linking EVA copolymers to provide higher molecular weight resins exhibiting reduced melt index. According to this process an EVA copolymer is cross-linked in a Brabender Plastograph under an inert atmosphere at a temperature of between 60.degree. and 250.degree. C. at atmospheric pressure employing an alkali metal salt or alkaline earth metal oxide, hydroxide or salt of a weak acid as cross-linking agent.
Each of the aforedescribed processes for cross-linking EVA copolymers is subject to one or more disadvantages. The processes of U.S. Pat. No. 3,491,075 and British Pat. Nos. 1,198,877 and 1,198,879 result in the by-product formation of an ester which must generally be removed from the cross-linked copolymer to provide a commercially acceptable product. The process of East German Pat. No. 94,489 appears to make mandatory the use of an inert atmosphere during the cross-linking operation. Such a requirement is undesirable from an operational standpoint. It is also known from U.S. Pat. No. 3,968,091 that mechanical working of an EVA copolymer by itself will result in a modestly reduced melt index. However, for practical reasons, it is desirable that a process be available for obtaining considerably greater reductions in melt index especially when the melt index of the starting copolymers is relatively high.